This invention relates generally to the production of hydrogen and, more particularly, to the production of hydrogen employing an adsorption system, whereby hydrogen may be produced with improved energy efficiency.
Hydrogen has a large number of uses. An increasingly important use is as a clean burning fuel in a combustion reaction wherein the only byproduct is water vapor. Similarly hydrogen is used as a reactant in fuel cells for the generation of electricity, again generating only water as a byproduct. Hydrogen is also widely used as a reactant in the production of many chemicals such as ammonia, ethanol and aniline, in hydrocracking, hydroforming and hydrofining of petroleum, in the hydrogenation of vegetable oils, in the hydrogenolysis of coal, as a reducing agent for organic synthesis and metallic ores, as a fuel for rocket engines, hypersonic aircraft and for missiles, and for many other uses.
The production of hydrogen is energy intensive and any improvement in energy efficiency in the production of hydrogen is desirable.
One important method for producing hydrogen is the production of synthesis gas and the subsequent separation and recovery of the hydrogen from the synthesis gas in a pressure swing adsorption (PSA) process. In the conventional steam-methane reformer (SMR) process, a mixture of high pressure steam and methane is passed through many tubes filled with reforming catalyst. The tubes are placed in a furnace and externally fired. Heat is transferred from the external tube surface by conduction through the tube wall and then by radiation and convection to the catalyst to provide the necessary heat for endothermic reforming reactions. Due to the indirect heat transfer method used in the process, expensive alloy tubes are required to withstand the temperature exceeding 1800 F. For efficient heat transfer the furnace must operate at significantly higher temperature, causing a high flue gas temperature and a large amount of sensible heat in the flue gas. The so called tail gas from the PSA process is used as a fuel for the furnace. However, due to the large heat requirement for the furnace a significant amount of additional fuel is required. In a typical arrangement a waste heat boiler is used to recover the sensible heat and to generate steam. Some of the steam is required in the process, but a significant fraction of steam must be exported for other uses. The SMR process is both capital and energy intensive and produces export steam that must be used for other applications.
The synthesis gas generated from the SMR process is cooled and sent to a water-gas shift reactor to shift the gas composition to that with a higher hydrogen and lower carbon monoxide concentration. The product gases from the shift reactor are further cooled and water is condensed out, and then sent to an adsorption bed of a PSA process for hydrogen recovery. The PSA is a cyclic regenerative process with multiple beds undergoing high pressure adsorption and low pressure desorption cycles to process a continuous syngas feed stream. The overall hydrogen generation scheme under the current practice is complex and capital intensive.
Accordingly, it is an object of this invention to provide a method for producing synthesis gas with improved energy efficiency and lower capital cost without export steam as compared with over known syngas production systems.
It is another object of this invention to provide an integrated method for the production of hydrogen from a feed stream which has improved overall energy efficiency and lower capital cost over presently known systems.
It is yet another object of this invention to provide an integrated method for the production of hydrogen from a feed stream which operates in a cyclic regenerative mode together with a PSA process with improved overall energy efficiency and lower capital cost over presently known systems.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for producing hydrogen comprising:
(A) reacting steam with a hydrocarbon feed stream in a heated regenerative reactor bed to produce hot synthesis gas and a cooled regenerative reactor bed, and cooling the hot synthesis gas in a gas cooler;
(B) passing the cooled synthesis gas through an adsorber containing adsorbent, adsorbing synthesis gas species other than hydrogen onto the adsorbent, and recovering hydrogen from the adsorber;
(C) desorbing adsorbed gas species from the adsorbent, and combusting the desorbed gas species with oxidant to produce hot combustion gas; and
(D) passing the hot combustion gas through the said cooled regenerative reactor bed to produce cooled combustion gas and said heated regenerative reactor bed.
As used herein the term xe2x80x9cregenerative bedxe2x80x9d means a container, typically refractory-lined, having input and output means and containing material, such as granular or shaped particles or monolithic honeycomb of various metals, alumina, magnesia, or zirconia-based ceramics, which is effective in storing and transferring heat.
As used herein the term xe2x80x9cregenerative heat recovery bedxe2x80x9d means a regenerative bed used solely for heat storage and transfer, including condensation or vaporization of water.
As used herein the term xe2x80x9cregenerative reactor bedxe2x80x9d means a regenerative bed which may also be used for carrying out a chemical reaction therein. Typically a regenerative reactor bed will also contain a chemical reaction catalyst as well as the heat transfer material.
As used herein the term xe2x80x9csynthesis gasxe2x80x9d or xe2x80x9csyngasxe2x80x9d means a fluid comprised essentially of hydrogen, carbon monoxide, water vapor, and carbon dioxide, partially reacted and unreacted hydrocarbon feed species and possibly also containing incidental impurities in minor amounts not affecting the properties of the fluid.